Strategies for walls
Ok, I found this video from the same guy:
https://www.youtube.com/watch?v=kZD8sSX7NtU
It uses booleans to cut out doors and windows. Seems to work for the above. Haven't tried it yet for slanted walls, but here's hoping it will do.
I was reluctant to use booleans first because they seemed so buggy. May not be buggy, just very hard to use whenever something is a little off. In my case, turned out one of my normals was pointing inwards and that was confusing everything.